Suction Pile Installation Method and Suction Pile For Use in Said Method

ABSTRACT

Suction pile installation method includes the steps of
         lowering a suction pile to the sea bed, while keeping a top end substantially closed,   providing openings in the suction pile near a lower end, the interior of the suction pile being in open fluid communication with the exterior via the openings,   dropping the pile onto the sea bed, while allowing air to escape from the pile via the openings,   allowing the suction pile to penetrate into the sea bed driven substantially by its weight over such a distance that the openings become closed from the exterior, and   pumping air from the suction pile for driving the pile further into the sea bed.

The invention relates to a suction pile installation method and to asuction pile for use in said method.

A suction pile is a thin walled steel cylinder, closed at least onelongitudinal end, which is placed on the sea bed with a lower end andwhich penetrates the sub sea soil with the aid of suction(under-pressure) created within the cylinder. The creation of thesuction can be with the aid of a suction source, such as a pump, that ismounted on, close to or at a distance (e.g. above the water surface,e.g. at a vessel) from the suction pile. The applied level of thesuction can be e.g. at least substantially constant, smoothly increaseor decrease or comprise a number of pulses. After use, the suction pilecan be removed by creating an overpressure within the cylinder, e.g. bypumping in (sea) water. Suction piles and their way of installing areknown from e.g. U.S. Pat. No. 4,318,641 in the name of Shell. The abovepatent describes a method and apparatus for securing a tubular elementto the bottom of the sea. It describes the steps of lowering the tubularelement from the water surface to the water bottom, removably securingthe pumping unit to the tubular element either before or after thelowering of the tubular element, closing the tubular element at theupper end, reducing the pressure within the tubular element by operatingthe pumping unit, so as to cause the tubular element to penetrate intothe water bottom to a desired depth. Subsequently, the operation of thepumping unit is stopped and the pumping unit is disconnected from thetubular element and raised to the water surface to recover it.

U.S. Pat. No. 6,719,496 in the name of Shell describes a method fordeploying a suction pile anchor in which the flood valves are opened onthe top of the suction pile and the suction pile anchor is off loadedfrom the anchor boat and lowered it to the sea floor. A remote operatedvehicle (ROV) with pump capacity is used to close the flood valves andto control the suction into the soil.

The known suction piles and installation methods employ valves on thetop of the suction pile in order to accommodate the lowering of thesuction pile during landing. The water pressure inside the pile rapidlyincreases while the pile is landing on the seabed when it is dropped(launched) from several meters above the seabed to penetrate into thesoil. The open valves or vents on the top of the pile allow the trappedwater to escape and therefore avoid building up a water pressure insidethe pile so that large pressure differentials between the inside andoutside of the pile are avoided. When the water cannot escape fastenough there will be a pressure build up that will cause a bearingcapacity failure in the soil when the pile lands.

After the pile is landed, the vents need to be completely sealed inorder to control the internal pressure. If the vents don't completelyseal, it is not possible to depressurise the pile to let it penetratethe soil, or to over pressurise it to come back out of the soil. It isneeded that each vent ensures a 100% sealing over the lifetime of aninstalled pile. Therefore the pile cover at the top end should be fittedwith specific installation aids. This makes the operation expensive asknown pile covers normally have 2 or more valves or costly closingdevices which requiring ROV assistance operations, particularly in deepwaters.

Normally a submerged pumping skid is attached to the vents and activatedby a ROV. The pumping skid is removed after the pile is depressurized.This operation also needs expensive anchor handling tug boats which areprovided with a heave compensation system to safely and smoothly landthe piles on the seabed.

It therefore is an object of the present invention to provide for asuction pile installation method and for a suction pile which is of arelatively simple construction and which avoids the use of complexventing valves at the top end.

It is furthermore an object of the present invention to provide asuction pile installation method which can be carried out using a simpleinstallation vessel.

It is again an object of the present invention to provide a suction pilewherein venting can be effectively controlled in a relatively simple andeconomic manner.

Hereto the installation method according to the invention ischaracterized by:

-   -   allowing fluid to escape from the pile via at least one opening        in the side surface, and    -   allowing the suction pile to penetrate into the sea bed over        such a distance that the at least one opening in the side        surface becomes closed off from the exterior by the soil, and    -   pumping air from the suction pile for driving the pile further        into the sea bed.

The suction pile has in its lower half, for instance in the first 2-4meters from the tip of the pile, an opening, such as two or more rows ofholes. These holes or other shaped openings will allow the water insidethe pile to escape during the first stage of the landing when thesuction piles touch down on the seabed.

During this stage no additional internal pressure will be build upinside the pile. In this way it is possible to reach (if required) acontrolled pile landing rate, for instance as high as 1 m/s.

As the opening, such as the row of holes in the side surface is in thelower segment of the pile, the first 2 to 3 m of seabed soil, which isanyway muddy, will be washed away by the water escaping from the holeswhile landing the pile. This disruption of the first mud line will haveno significant influence on the pile holding capacity of the soil. Asthe falling pile penetrates the mud line by its own weight, the pilewill penetrate also the soil under the mud line which soil will thencover the holes. After this free fall penetration of the pile into thesoil, the pile can be moved further into the soil with the known processof adding an under-pressure within the pile by disconnectable vacuumpumps at the top of the pile activated by a ROV or via a suction hosewhich is attached to the top of the pile.

The openings in the suction pile can replace the vents on the pile coverexcept for a small valve, which is used for the suction process (whilecreating an under pressure for pile penetration or an overpressure forthe breakout, if needed). Therefore there no longer is a need forproviding complicated and costly closing devices that are operated witha ROV manipulation system.

The pile according to the present invention with one or more openings inthe lower side surface section can be easily installed with a knownanchor handling tug boat which does not need to be equipped with a heavecompensation system or a constant tension devise which normally isrequired when a control line connects the venting valves on the pilewith the vessel. This latter is especially relevant for deep waterprojects were the suction piles and hence the heave compensation devicesneed to be relatively large and expensive to be able to reach to theseabed.

The at least one opening in the side surface of the suction pileaccording to the present invention has furthermore been found to ensurea controlled vertical landing without a large inclination of the pilewhich ensures an optimal bearing capacity in the soil.

An embodiment of a suction pile in accordance with the present inventionwill be explained in detail with reference to the accompanying drawings.In the drawings:

FIG. 1 shows a side view and a top view respectively of a suction pileknown from the prior art,

FIG. 2 shows a side view and a top view respectively of a suction pileaccording to the present invention, and

FIG. 3 shows a side view of a suction pile attached to a removableROV-operated pump skid.

FIG. 1 shows a suction pile 1 with a top surface 2, a cylindrical sidesurface 3, and an open bottom surface 4. The top surface comprises fourventing valves 6,6′ and an attachment eye 5 for attaching to an anchorcable and/or to a lowering cable upon installation. Upon penetration ofthe bottom surface 4 into the sea bed, water is vented via the valves6,6′. Upon creating an under pressure inside the suction pile, forintroduction into the sea bed, the valves 6,6′ are closed. A pump or airhose is during creation of an under pressure, connected via one of thevalves 6,6′.

In FIG. 2 it can be seen that in the suction pile 10 according to theinvention, only a single air connection, or valve or vent 12 is made inthe top surface 11. As can be seen in FIG. 3, the valve 12 can beconnected to a removable suction pump skid 21 which is activated by aROV 22. Alternatively the vent or valve 12 can be directly connected toa surface vessel 14. In the lower halve L of the side surface 15, tworows of openings 16, 17 is present, which provide a venting path viawhich water can leave the pile 10 when the bottom surface 18 enters intothe soil of the sea bed 19. After the holes 16,17 have been closed offby the soil, an under pressure is created inside the pile 10 via the airhose 13, such that the pile is inserted into the soil substantially overits whole length for anchoring a floating structure to the sea bed.

1. Suction pile installation method comprising the steps of: lowering asuction pile (10) to the sea bed, the suction pile having a top surface(11), a substantially cylindrical side surface (15) and a bottom surface(18) which is at least partially open, keeping the interior of thesuction pile in open fluid communication with the exterior at least viathe bottom surface, dropping the pile onto the sea bed (19),characterized in that the method comprises: allowing fluid to escapefrom the pile via at least one opening (16,17) in the side surface (15),and allowing the suction pile to penetrate into the sea bed over such adistance that the at least one opening (16,17) in the side surfacebecomes closed off from the exterior by the soil, and pumping air fromthe suction pile for driving the pile further into the sea bed. 2.Method according to claim 1, wherein the pile (10) is driven into thesoil over the distance that the at least one opening (16,17) becomesblocked substantially by its own weight.
 3. Method according to claim 1,wherein the pile (10) is connected to a removable disconnectable pumpskid which is activated by a ROV.
 4. Method according to claim 1,wherein the pile (10) is connected via an air hose (13) to a surfacevessel (14).
 5. Method according to claim 1, wherein the suction pile(10) is applied from a surface vessel (14) and is attached to saidvessel via an elongate member (13) without the use of a tensioncompensation device.
 6. Suction pile (10) comprising a top surface (11),a substantially cylindrical side surface (15) and a bottom surface (18)which is at least partially open, characterized in that the suction pilecomprises in a lower half (L) of the side surface at least one opening(16,17).
 7. Suction pile (10) according to claim 6, the suction pilecomprising in an upper halve a valve (12) for a relatively slow rate offluid transfer, no fluid venting means being present in the upper halvefor fluid transfer from the pile upon entering the soil.
 8. (canceled)9. Method according to claim 2, wherein the pile (10) is connected to aremovable disconnectable pump skid which is activated by a ROV. 10.Method according to claim 2, wherein the pile (10) is connected via anair hose (13) to a surface vessel (14).
 11. Method according to claim 3,wherein the pile (10) is connected via an air hose (13) to a surfacevessel (14).
 12. Method according to claim 2, wherein the suction pile(10) is applied from a surface vessel (14) and is attached to saidvessel via an elongate member (13) without the use of a tensioncompensation device.
 13. Method according to claim 3, wherein thesuction pile (10) is applied from a surface vessel (14) and is attachedto said vessel via an elongate member (13) without the use of a tensioncompensation device.
 14. Method according to claim 4, wherein thesuction pile (10) is applied from a surface vessel (14) and is attachedto said vessel via an elongate member (13) without the use of a tensioncompensation device.